One of the classic scientific debates, on a par with "nature vs. nurture," albeit far more obscure, is the question of what caused the great megafaunal extinction at the end of the Pleistocene.

Pretty much everywhere you look, you find evidence of big critters roaming the earth - mastodons, mammoths, big camels, and my favorite, beavers the size of black bears. And then they "blink out," to borrow a lovely phrase I heard a biologist use recently. In a very short period of time in geologic terms, they're gone.

Two dominant explanations are offered. The extinction coincides with the end of the Pleistocene, the emergence from the last ice age 10,000 years ago. That's big-time climate change. Could that have killed them off? But there's a second earth-changing event that happens around that same time - the explosion of us. Could we have killed them?

There's a new review paper(sub. req.) in Friday's Science by Barnosky et al. that offers a classically equivocal answer - it's both! And more research is needed!

OK, that's cheap, it's really an interesting paper.

The authors argue: "Evidence from paleontology, climatology, archaeology, and ecology now supports the idea that humans contributed to extinction on some continents, but human hunting was not solely responsible for the pattern of extinction everywhere. Instead, evidence suggests that the intersection of human impacts with pronounced climatic change drove the precise timing and geography of extinction in the Northern Hemisphere."

For example, pretty much all of the survivors were nocturnal and lived in the woods, where it would have been harder to hunt them. That argues for the human hunting hypothesis. Except in Africa, where there were lots of people, but large, slow-breeding animals continued to live in open country. Think elephants. And in northern latitudes, the timing of the major extinction events coincides quite closely with climate changes.

The archaeological evidence is ambiguous as well. Our ancestors on this continent, for example, seemed to be chowing down on lots of mammoth, but didn't have much taste for the ancient peccary known as Platygonus.

The good thing is that we finally know enough, and have developed finer-grained techniques, to begin to tease out the details of the complex interaction between climate and humanity's role as a major predator in the early Holocene ecosystem, which should allow us to begin understanding the role of each in the complex changes that overtook our planet 10,000 years ago.